Development within offshore oil and gas exploration in the recent years has been directed to subsea installations for processing and transport of oil and gas. The subsea installation is located at the well on the seabed and is connected to a floating structure by means of a so-called riser, which constitutes a conduit between the subsea installation and the floating structure. The subsea installation may be a wellhead or other type of equipment positioned on the seabed or in a fixed position above the seabed. The floating structure may for instance be a vessel. Movements of the floating structure due to waves, wind and sea-currents can cause bending of the riser and influence the tension thereof. The riser is designed to be capable of withstanding a certain amount of bending and tension encountered during normal conditions. However, in an emergency situation, for instance due to severe weather conditions, the riser might be subjected to excessive bending and tensioning with a risk of damage to the riser and to the equipment connected to the riser. In order to avoid such damage, a so-called emergency disconnect package can be used to disconnect the riser from the subsea installation in an emergency situation or if the operator predicts that adverse conditions are imminent. The emergency disconnect package is traditionally secured to the subsea installation below a stress joint provided at the lower end of the riser. Such an arrangement is for instance disclosed in U.S. Pat. No. 6,659,690 B1 and WO 2008/051092 A1. When the riser has to be disconnected from the subsea installation in an emergency situation, the emergency disconnect package is actuated to release the stress joint and the riser from the subsea installation. Normally, the riser also comprises a safety joint provided with a weak link that will automatically break when subjected to an excessive axial force. Such a safety joint may be located at the base of the stress joint or connected between the stress joint and the lowermost riser joint or between two riser joints at the lower end of the riser.
The safety joint of the present invention is particularly intended to be used in a completion and work over riser. A completion and work over riser is used in the oil and gas industry when oil and/or gas is to be extracted from one or more offshore wells. Completion and work over operations are performed on a subsea wellhead using a completion and work over riser. A completion and work over riser may for instance be used for installing or retrieving a so-called X-mas tree. It may also be used for installing or pulling a so-called tubing hanger. With a dual bore riser it will be possible to circulate a fluid down through the production pipe and up through the annulus pipe or vice versa. Such fluid circulation may be used to clean a well and to test and verify a circulation path. The bore of the production pipe and the bore of the annulus pipe of a dual bore riser may be connected to two corresponding bores in an X-mas tree so that a wire line or coiled tubing can be used to access plugs or other devices installed in the bores of the X-mas tree. The bore of the production pipe of a riser may also be connected to the production tubing that extends from a tubing hanger all the way to the bottom of a well. Installing the tubing and tubing hanger is referred to as completing a well and is consequently a completion operation. When a well is completed, it is made ready for production of oil and/or gas or alternatively for injection of gas or water. If the well does not produce as expected, it may be overhauled or repaired in different ways. This is referred to as work over.
A completion and work over riser may be of the monobore type or the dual bore type. A dual bore riser comprises a production pipe and an annulus pipe extending in parallel with the production pipe. The production pipe is typically designed for taking a load and has strength for lifting, whereas the annulus pipe may be a pressure containing pipe with no strength for lifting. A monobore riser comprises a production pipe but no annulus pipe.